Fractography Reveals Machining G.D. Quinn, L.K. Ives and S. Jahanmir National Institute for Standards and Technology, Gaithersburg, Md. A machining damage map that summarizes the effect of grinding wheel grit and grinding direction on the strength of a finished silicon nitride has been created. Diamond abrasive wheel grinding creates machining cracks that can control strength of ground ceramics. Knowledge of the size and severity of machining cracks facilitates the choice of optimum machining practices for component machining. This knowledge also aids specifications for specimen preparation in standard test methods. 1 Unfortunately, machining cracks often are difficult to detect in ceramics with enhanced fracture toughness from interlocking grain microstructures that create rough fracture surfaces. This study characterized machining cracks in hundreds of ground rods and bend bars in ceramics and glasses and a sintered reaction-bonded silicon nitride (SRBSN) in particular. We learned that parallel machining cracks leave telltale mark- ings on fracture surfaces that can be easily detected using simple fractographic techniques. Crack size and shape depended primarily on the direction of grinding and the grind- ing wheel grit size. In some instances, specimens broke from the material’s inherent flaws, and machining damage had no effect on strength. This article is a summary of important findings from a com- prehensive NIST Special Publication 2 and from an overview paper that appeared in a recent issue of Ceramic Engineering and Science Proceedings. 3 Material and Testing Procedures Although a variety of ceramics and glasses were studied, the primary focus was on one SRBSN (Ceralloy 147-31N, Ceradyne, Cosa Mesa, Calif.). This SRBSN was the same material evaluat- ed in a recent study of 320 vs 600 grit longitudinally ground standard ASTM C 1161 four-point bend bars. 1,4 Sets of 10 or 30 rod or bar specimens were prepared and tested per grinding condition. All cylindrical rods were 6 mm in diameter by 100 mm in length. Rods were transversely or longitudinally ground with resin-bonded diamond abrasive wheels with 150 to 600 grit sizes. Rods were tested on a four- point flexure fixture with 40 80 mm spans. Fractography Reveals Machining C r a c k s C r a c k s © The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org July 2003 9101
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Fractography Reveals Machining
G.D. Quinn, L.K. Ives and S. JahanmirNational Institute for Standards and Technology,Gaithersburg, Md.
A machining damage map that summarizes
the effect of grinding wheel grit and grinding
direction on the strength of a finished silicon
nitride has been created.
Diamond abrasive wheel grinding creates machining cracksthat can control strength of ground ceramics. Knowledge of thesize and severity of machining cracks facilitates the choice ofoptimum machining practices for component machining. Thisknowledge also aids specifications for specimen preparation instandard test methods.1 Unfortunately, machining cracks oftenare difficult to detect in ceramics with enhanced fracturetoughness from interlocking grain microstructures that createrough fracture surfaces.
This study characterized machining cracks in hundreds ofground rods and bend bars in ceramics and glasses and asintered reaction-bonded silicon nitride (SRBSN) in particular.We learned that parallel machining cracks leave telltale mark-ings on fracture surfaces that can be easily detected usingsimple fractographic techniques. Crack size and shapedepended primarily on the direction of grinding and the grind-ing wheel grit size. In some instances, specimens broke fromthe material’s inherent flaws, and machining damage had noeffect on strength.
This article is a summary of important findings from a com-prehensive NIST Special Publication2 and from an overviewpaper that appeared in a recent issue of Ceramic Engineeringand Science Proceedings.3
Material and Testing ProceduresAlthough a variety of ceramics and glasses were studied, theprimary focus was on one SRBSN (Ceralloy 147-31N, Ceradyne,Cosa Mesa, Calif.). This SRBSN was the same material evaluat-ed in a recent study of 320 vs 600 grit longitudinally groundstandard ASTM C 1161 four-point bend bars.1,4
Sets of 10 or 30 rod or bar specimens were prepared andtested per grinding condition. All cylindrical rods were 6 mm indiameter by 100 mm in length. Rods were transversely orlongitudinally ground with resin-bonded diamond abrasivewheels with 150 to 600 grit sizes. Rods were tested on a four-point flexure fixture with 40 � 80 mm spans.
Fractography Reveals MachiningCracksCracks
©The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org July 2003 9101
Machining Cracks Revealed by Fractography
All rectangular bars were “B” sized(3 � 4 � 45 mm) four-point flexurespecimens prepared in accordancewith ASTM C 1161. Specimens wereeither longitudinally or transverselyground with 80 to 600 grit wheels.The rods and bars were different insize, but the Weibull effectivevolumes and effective surfaces wereactually similar. Additional detailsare in Ref. 2.
The fractographic analysis wasperformed in accordance with ASTMC 1322.5 All fracture surfaces wereexamined using a binocular stereo-microscope at magnifications up to
Schematic of flaws introduced by machining or scratching a ceramic or glass surface. Adapted from Fig. 1 in Rice and Mecholsky.6
Orthogonal cracks
Abrasiveparticle
Parallel cracks
Containercrack
Straightcrack
Bowedcrack
Offsetcracks
Longitudinally ground specimens
Flexure testing activates theorthogonal machining cracks.
Transversely ground specimens
Flexure testing activates theparallel machining cracks.
Optical stereomicroscopy was effective indetecting the telltale signs of machiningdamage, especially with low-incident-angle(vicinal) illumination.
Viewingdirection
Illumination source
Origin
Fracturesurface
Test piece
“Zipper crack” is one form of parallel machining crack in a ground bar.Machining crack hackle steps between crack segments are a telltale featurethat is accentuated by vicinal illumination. These cracks are easily detectedeven at low magnifications when vicinal illumination is used.
Machining cracks in ground rods.
Lowfeedrate
Highfeedrate
Spiral“barber pole”
pattern
Flat, coplanar parallel crack V machining crack
©The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org July 2003 9102
205�. A large subset was examined using a scanning electronmicroscope. Optical microscopy was effective in finding andcharacterizing the machining cracks once we learned to recog-nize their telltale signs.
It was essential to illuminate the specimen fracture surfacefrom the side with a bright, low-incident-angle (vicinal) illumi-nation source. The vicinal illumination accentuated manytelltale features, such as machining crack hackle. As the studyprogressed and we gained greater experience in detecting thetelltale features of machining damage cracks, we could immedi-ately identify parallel cracks from transverse grinding withinseconds during the first inspection using the stereomicroscope.
Wheel Grit Size Controls Crack DepthAlthough grinding parameters were varied in the study, the datashowed that wheel grit size controlled crack depths. Inherentstrengths, whereby the fracture origins were inherent flaws,such as agglomerates or inclusions, were measured on 320 gritlongitudinally ground rods. Rods ground transversely using 600grit either matched or approximately matched the inherentstrengths, because the small 10–20 µm machining cracks weresmaller than or less severe than the inherent flaws.
The size of the strength-controlling machining cracks variedby as much as a factor of two for any given batch of identicallyground test pieces. This size variability was matched by a 40%variation in strength within a batch when machining flaws werestrength limiting. Thus, contrary to expectations, there wassome variability in the severity of machining cracks in identi-cally ground specimens.
Depth of machining cracks in rods and bars depended primarily upon the diamond wheel grit.
Machining damage depths for silicon nitride.
Grinding wheel
600 grit wheel14–20 µm deep cracks(0.0005–0.0008 in.)
320 grit wheel14–40 µm deep cracks(0.0006–0.0016 in.)
220 grit wheel25–55 µm deep cracks(0.0010–0.0022 in.)
150 grit wheel30–70 µm deep cracks(0.0012–0.0028 in.)
80 grit wheel35–80 µm deep cracks(0.0014–0.0032 in.)
©The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org July 2003 9103
Machining Cracks Revealed by Fractography
References1B. Mikijelj and R. Allor, “Effects of
Machining on Si3N4 Strength,” Am. Ceram.Soc. Bull., 82 [4] 57–60 (2003)
2G.D. Quinn, L.K. Ives and S. Jahanmir, Onthe Fractographic Analysis of MachiningCracks in Ground Ceramics: A Case Studyon Silicon Nitride, NIST Special Publication966. National Institute of Standards andTechnology, Gaithersburg, Md., May 2003.
3G.D. Quinn, L.K. Ives and S. Jahanmir,“Machining Damage Cracks: How to Find andCharacterize Them by Fractography.” Ceram.Eng. Sci. Proc., (2003).
4“Standard Test Method for FlexuralStrength of Advanced Ceramics at AmbientTemperature,” ASTM Designation C 1161-02.Annual Book of Standards, Vol. 15.01.American Society for Testing and Materials,West Conshohocken, Pa., 2002.
5“Standard Practice for Fractography andCharacterization of Fracture Origins inAdvanced Ceramics,” ASTM Designation C1322-02. Annual Book of Standards, Vol.15.01. American Society for Testing andMaterials, West Conshohocken, Pa., 2002.
6R.W. Rice and J.J. Mecholsky, “The Natureof Strength-Controlling Machining Flaws inCeramics”; pp. 351–78 in The Science ofCeramics Machining and Surface FinishingII, NBS Special Publication 562. Edited byB.J. Hockey and R.W. Rice. National Bureauof Standards, Gaithersburg, Md., 1979.
A review of all published machining crack-size data for siliconnitrides revealed that most data fit the same trend, but thattoughened silicon nitrides actually had deeper machining crackson average than in untoughened silicon nitrides. This paradox isaccounted for and discussed in detail in Ref. 2.
Another surprise was that the fracture origin was alwaysassociated with a single striation in several specimen batches.The striation was not the most obvious or most severe inappearance, but its damage dominated strength. It is believedthat a single “renegade” grit in the grinding accounted for thestriation.
Different crack damage depths occurred from different grind-ing wheels. These depths should be considered by machinistsduring final finishing if they wish to eliminate machining cracksfrom prior finishing steps or by users concerned about residualdamage after final finishing.
On the basis of our results and a review of the literature, weprepared a machining damage map that summarizes the effectof grinding wheel grit and grinding direction on the strength offinished silicon nitride in general.
Over 100 illustrations of machining cracks are included in Ref.2. Parallel cracks are much easier to find now that the telltalefeatures have been identified, but orthogonal cracks remain achallenge. ■
Editor’s NoteCertain commercial materials or equipment are identified in this article to
specify adequately the experimental procedure. Such identification does notimply endorsement by NIST nor does it imply that these materials or equipmentare necessarily the best for the purpose.
Machining damage map for silicon nitride.
©The American Ceramic Society American Ceramic Society Bulletin www.ceramicbulletin.org July 2003 9104
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